Pertinent areas of the classification manual concerned with this type of invention are, among others, Class 208, Subclass 310 and Class 585, Subclasses 820, 701 and 738.
In Gray Jr. et al, U.S. Pat. No. 4,476,345, an invention is disclosed in which a portion of one of the product streams in an isomerization process is used to wash a recycle gas stream to improve the quality of the isomerate products. The molecular sieve adsorbent of Gray is one which can be naturally occurring or synthetically produced comprising a three-dimensional crystalline-zeolitic aluminosilicate which will selectively, on the basis of molecular size of the pores, adsorb normal paraffins from the isomerized product from branched chained and/or cyclic paraffins. The molecular sieves have pore diameters of about 5A and are exemplified by a calcium 5A zeolite which exhibits pore diameters ranging from about 3 to about 5 Angstroms.
The Gray et al disclosure is an improvement upon an isomerization process as taught in Holcombe, U.S. Pat. No. 4,210,771. This is a process for the virtual complete isomerization of normal paraffin hydrocarbons in a feed stream consisting essentially of mixed normal and branched hydrocarbons, where the feed stream is passed first through an isomerization reactor and the products derived therefrom are passed to an adsorption section which separates normal from branched paraffins to form an isomerate having both di- and mono-branched paraffins. A recycle stream comprising nearly pure normal paraffins is usually recycled to exhaustion. Other disclosures which may be commensurate with Holcombe comprise U.K. Pat. No. 876,730 and U.S. Pat. No. 3,755,144 issued to Asselin.
The zeolite molecular sieve employed in Gray et al and Holcombe may be selected from any adsorbent which selectively adsorbs normal paraffins based on the molecular pore size of the aluminosilicate. Particularly suitable zeolites of this type are calcium exchanged zeolite 5-A. Naturally occurring zeolite molecular sieve which could be substituted for calcium 5-A zeolite include chabazite and erionite. The particular flow scheme of adsorption as taught by Holcombe '771 is herein incorporated by reference to show an operable multiple zeolitic molecular sieve adsorption means, to achieve proper adsorption-fill and desorption-purge. The Holcombe patent is completely silent as to arrangements of a multiple number of different sieves which may be present in the absorption separation technique. In fact, in the drawing of Holcombe, the adsorption bed systems, 44, 46, 48, and 50, are all comprised of calcium 5A zeolite in the form of 1/16-inch cylindrical pellets. Branched paraffins, whether they be mono- or di-branched, flow through the adsorption bed while unbranched normal paraffins are adsorbed. After a purge of the adsorbed normal paraffins from the zeolite molecular sieve, the recycle stream is comprised nearly entirely of normal paraffins and recycle hydrogen. This is mixed with the incoming feed before charge to the isomerization zone. The placement of these types of molecular sieves upstream of isomerization will result in only normal paraffins being passed to the isomerization zone while mono-methyl-branched paraffins, in admixture with the more desirable di-branched paraffins, will not be captured and, therefore, will not be further isomerized into the more valuable di-branched paraffins.
A second Holcombe patent, U.S. Pat. No. 4,176,053, discusses a normal paraffin-isomerization separation process. By this technique, normal paraffins are isolated from a feedstock mixture comprising normal and branched paraffins at super atmospheric pressures using an adsorption system comprising at least four fixed adsorbent beds containing a calcium 5A molecular sieve. A stream is formed comprising vapor from void space purgings of the adsorbent and feedstock containing iso-paraffins and normal paraffins. The molecular sieve employed to separate normal paraffins from said stream is selected to adsorb only normal paraffins from a mixture of branched, cyclic and normal hydrocarbons.
In U.S. Pat. No. 3,836,455 issued to Blytas, the separation of methylpentane and 2,2-dimethylbutane (as contrasted with 2,3-dimethylbutane of the instant invention) is accomplished using an offretite zeolite. U.S. Pat. No. 4,251,499 issued to Nanne et al teaches that ferrierite sieves are effective for dividing substantially unbranched structures (n-paraffins) from mixtures of same with branched structures (both mono-methyl and di-branched paraffins). Such was the state of the art in 1981 although the instant invention has shown that this teaching is no longer accurate in regard to the adsorption capacity of ferrierite aluminosilicates as defined herein.
These patents teach that it is most advantageous to recycle normal paraffins to thereby isomerize the same to the isomerate components comprising mono-methyl-branched paraffins and di-branched paraffins. These disclosures suggest that the isomerate will have a certain quantity of mono-methyl-branched paraffins derived from the isomerization zone. These mono-methyl-branched paraffins will have an inherently lower octane value than the di-branched paraffins whether or not they are mixed with the more preferred di-branched paraffins before or after isomerization.
In contrast, applicants have discovered a new and more efficient isomerization process utilizing a pre-isomerization molecular sieve whereby both normal paraffins and mono-methyl-branched paraffins are passed to isomerization with little or no presence of di-branched paraffins. Using the specific pre-isomerization molecular sieve separation technique of this process, mono-methyl-branched paraffins are diminished in the refinery gasoline pool while di-branched paraffins are both preserved and produced. In other words, this process increases the degree of branching in the ultimate refinery gasoline pool by improving the effectiveness of the isomerization step